[Feature Request] El-Alert to support html description

### Existing Component
Yes

### Component Name
el-alert

### Description
I would like to be able to specify that the description will need to be treated as html.
Though there is a workaround which you can put the description in the el-alert default slot, wrapped in an element and inside a v-html directive.

Semantically and on the user friendly side I think it would made more sense to have the option on the component rather than doing this.